.TH std::random_device 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::random_device \- std::random_device

.SH Synopsis
   Defined in header <random>
   class random_device;        \fI(since C++11)\fP

   std::random_device is a uniformly-distributed integer random number generator that
   produces non-deterministic random numbers.

   std::random_device may be implemented in terms of an implementation-defined
   pseudo-random number engine if a non-deterministic source (e.g. a hardware device)
   is not available to the implementation. In this case each std::random_device object
   may generate the same number sequence.

.SH Member types

   Member type         Definition
   result_type \fI(C++11)\fP unsigned int

.SH Member functions

.SH Construction
   constructor       constructs the engine
   \fI(C++11)\fP           \fI(public member function)\fP
   operator=         the assignment operator is deleted
   (deleted) \fI(C++11)\fP \fI(public member function)\fP
.SH Generation
   operator()        advances the engine's state and returns the generated value
   \fI(C++11)\fP           \fI(public member function)\fP
.SH Characteristics
   entropy           obtains the entropy estimate for the non-deterministic random
   \fI(C++11)\fP           number generator
                     \fI(public member function)\fP
   min               gets the smallest possible value in the output range
   \fB[static]\fP \fI(C++11)\fP  \fI(public static member function)\fP
   max               gets the largest possible value in the output range
   \fB[static]\fP \fI(C++11)\fP  \fI(public static member function)\fP

.SH Notes

   A notable implementation where std::random_device is deterministic in old versions
   of MinGW-w64 (bug 338, fixed since GCC 9.2). The latest MinGW-w64 versions can be
   downloaded from GCC with the MCF thread model.

.SH Example


// Run this code

 #include <iostream>
 #include <map>
 #include <random>
 #include <string>

 int main()
 {
     std::random_device rd;
     std::map<int, int> hist;
     std::uniform_int_distribution<int> dist(0, 9);

     for (int n = 0; n != 20000; ++n)
         ++hist[dist(rd)]; // note: demo only: the performance of many
                           // implementations of random_device degrades sharply
                           // once the entropy pool is exhausted. For practical use
                           // random_device is generally only used to seed
                           // a PRNG such as mt19937

     for (auto [x, y] : hist)
         std::cout << x << " : " << std::string(y / 100, '*') << '\\n';
 }

.SH Possible output:

 0 : ********************
 1 : *******************
 2 : ********************
 3 : ********************
 4 : ********************
 5 : *******************
 6 : ********************
 7 : ********************
 8 : *******************
 9 : ********************
